1. The Origins of Life
Dr. Rachel Mackelprang
Book: pg 10-24
2. Origins of life: central questions
•How many times did life evolve?
•How is life related (what is the structure of the tree of
life)?
•How can we figures this out?
3. The origins of life
a b c d e f g h
Consider the default class tree
4. The origins of life
a b c d e f g h
If this tree included all major group of organisms…
What does that imply about the number of times
life evolved?
5. The origins of life
a b c d e f g h
Consider the default class tree
MRCA
6. The origins of life
a b c d e f g h
Consider the default class tree
MRCAThis implies a single
common ancestor to all
life, sometime before the
MCRA
And thus, that
there is one “tree
of life”
7. What if life originated twice?
a b c d e f g h
What would you expect of a “tree of life” if there
were 2 origins?
8. What if life originated twice?
a b c d e f g h
No theoretical reason why this couldn’t
have occurred.
Origin 1
Origin 2
9. How many trees of life?
• How do we dis)nguish the two hypotheses?
1. A single origin (and thus a single tree of life)
2. Mul)ple origins (and thus mul)ple trees)
• What data might support one hypothesis over
the other?
10. Universal homologies imply one origin
•Characters that our found in the same state in all
organisms are considered universal
•Character (i.e., features like the smiley face) that are
inherited from a common ancestor are
homologous
•Homologous characters that are found in the same
state in all organisms are universal homologies
11. Universal homologies: examples
DNA, double stranded, w/ ACTG
RNA w/ ACUG
Translated into proteins
• using tRNA
• using ribosomes
• using 3 letter code
• using 20 core amino acids
Lipoprotein cell envelope
Central dogma: from gene to protein
12. Universal homologies: examples
• Molecular and cellular features
–Use of DNA as gene)c material
–Use of ACTG in DNA
–Use of ACUG in RNA
–Three leHer gene)c code
–Central dogma (DNA->RNA->protein)
–20 core amino acids in proteins
• Specific complexes and genes
–Ribosomal proteins and RNA
–RNA polymerase proteins
24. How to build a tree of life?
• For building a single tree including all organisms
• Need traits that are homologous between all
organisms but which have variable states
• Examples???
–Un)l the 1960s these were hard to come by
–Researches focused instead on rela)onships within par)cular
branches of the Tree of Life
–Within sub-branches, more traits were available to use (e.g.,
bones in mammals)
26. Ernst Haeckel 1866
Reflects some important understandings
Species evolve by diversification, not progression
Eukaryotes didn’t evolve from bacteria
Animals didn’t evolve from ciliates
Plants didn’t evolve from Fungi
Humans didn’t evolve from Chimps
These share a common ancestor from which
each diverged
29. How did he do it?
Recall the need for universal homologies
30. How did he do it?
Used molecular information from ribosomes
31. rRNA systematics
• All cellular organisms have ribosomes
• All have homologous subunits of the ribosomes including specific
ribosomal proteins and ribosomal RNAs (i.e., these are universally
homologous genes)
• Woese determined the sequences of ribosomal RNAs from different
species
• The sequences are highly similar but have some varia)on
• Each posi3on in a rRNA can be considered a dis)nct character trait
• Each posi)on has mul)ple possible character states (A,C,U,G)
Used the rRNA Sequences to build the tree of life, revealing a “hidden”
major branch!
